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Return Path challenges accepted subject line wisdom: Personalization boosts open rates, with the exception of movies, music, gaming and retail, where it actually lowers them. 'Free': Use it with abandon. No measured impact on deliverability. ALL CAPS: DITTO.

A number of firms that have access to aggregate results data in the marketing email space have attempted to analyze this data with varying degrees of success. Return Path, an email intelligence firm that also hosts the Sender Score blacklist, is one of them and recently posted these three observations to their blog.

Personalization works only some of the time. Most of the time, Return Path saw a statistically significant difference, especially in the Hotel, Health and Beauty, and Home categories.

However, for Music, Movies, and Games and Retail, the use of a subscriber's first name actually hurt the open rates. They theorized that subscribers are likely to have more personal connections with some brands or categories of senders than others. If you offer some kind of loyalty programs, you can likely benefit from the use of personalization more than if you are promoting games, movies or music.

"Free" does not automatically get your email sent to spam. There is a widespread perception that using the word "free" in a subject line is going to result in an email being marked as spam by Mailbox Providers. Return Path found that for most of the categories, there is only a slight variation in inbox placement rates for subject lines that contain the word "free", with the exception of the Jewelry category. If you're concerned about Inbox Placement Rates, using the word "free" in your subject lines is likely not your problem.

ALL CAPS. ALL OK. Another common perception is that subject lines with all capital letters will also make it more likely your email is marked as spam. When Return Path looked at senders that did use all capital letters in their subject lines, the use of all caps did not have a negative affect on Inbox Placement Rates. That doesn't of course mean that it is a good idea; recipients may perceive it as spam which is nearly as bad! Test it thoroughly and see if it works for you.